FIX: Clean required category tag groups with invalid tag_group ids (#16414)

The category table's required_tag_group_id contained references to deleted tag groups, which we copied to the new table. The new serializer tries to get the associated tag group name but fails because the tag group is nil.

This PR adds an inner join in the original migration to make sure tag groups still exist and adds a new post-migration to fix already migrated sites.

db/migrate/20220401130745_create_category_required_tag_groups.rb

@@ -17,7 +17,8 @@ class CreateCategoryRequiredTagGroups < ActiveRecord::Migration[6.1]
       (category_id, tag_group_id, min_count, updated_at, created_at)
       SELECT c.id, c.required_tag_group_id, c.min_tags_from_required_group, NOW(), NOW()
       FROM categories c
-      WHERE c.required_tag_group_id IS NOT NULL
+      INNER JOIN tag_groups tg ON tg.id = c.required_tag_group_id
+      WHERE tg.id IS NOT NULL
     SQL
   end
 

db/post_migrate/20220407195246_remove_category_required_tag_groups_without_tag_groups.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class RemoveCategoryRequiredTagGroupsWithoutTagGroups < ActiveRecord::Migration[6.1]
+  def up
+    execute <<~SQL
+      DELETE FROM category_required_tag_groups
+      WHERE id IN (
+        SELECT crtg.id
+        FROM category_required_tag_groups crtg
+        LEFT OUTER JOIN tag_groups tg ON crtg.tag_group_id = tg.id
+        WHERE tg.id IS NULL
+      )
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end